Activation-induced cytidine deaminase (AID) initiates immunoglobulin gene somatic hypermutation, gene conversion and class switch recombination (CSR) by inflicting DNA lesions at specific genomic loci via DNA cytosine deamination (that converts cytosine to uracil). While these lesions are essential for the production of optimized antibodies against infections, they are also threats to the genome integrity. AID-associated oncogenic mutations and chromosomal translocations are frequently seen in tumors of B cell origin. We propose three specific aims to address several significant gaps in our knowledge about the mechanism of CSR. In aim 1, we will manipulate the transcription control elements (promoters/enhancers) at IgH ? locus in CH12F3 cells to identify cis-acting DNA elements critical for targeting AID to switch (S) regions. In aim 2, we will determine the frequency and precise positions of AID footprints in S regions to delineate AID actions in vivo during CSR. In aim 3, we will determine which DNA ligase is responsible for alternative end-joining of S region breaks.